1. Field of the Invention
The present invention relates to a lithium polymer battery and a manufacturing method for the same and, more particularly, to a stacked type lithium polymer battery and a manufacturing method for the same.
2. Description of the Related Art
Matrix materials such as carbon materials and conductive polymers which utilize a dope-undope process of lithium ions instead of utilizing lithium metal or metal alloy thereof for a negative electrode have been developed in recent years. Accordingly, generation of dendrite (which occurs in the case where lithium metal or metal alloy thereof is utilized) no longer theoretically occurs. Therefore, short circuit problems inside of the battery has been considerably reduced. In particular, it is known that the dope-undope potential of lithium of carbon materials is closer to the deposition-dissolution potential of lithium than that of other materials. Specifically, graphite materials are carbon materials that can theoretically hold lithium in their crystal lattice at a rate of one lithium atom relative to six carbon atoms and which have a high capacity per unit weight and per unit volume. Furthermore, the potential of intercalation-deintercalation of lithium is fiat in graphite materials which are chemically stable so as to greatly contribute to the cycle stability of batteries.
As a result of such research and development, a so-called lithium ion battery employing a carbon material as the negative electrode has been commercialized and has rapidly come into wide use as a power source for mobile devices. Such battery is lightweight and has a high capacity which is utilized to the fullest.
In addition, ion conductive polymers having high ion conductivities have been recently reported. Research has been directed to increase the prevention of liquid leakage, the safety level, and the extended shelf life of batteries in the case where liquid electrolytes are used.
Normal (straight) chain polymers, net crosslinked polymers and comb polymers of homopolymers or copolymers having basic units of ethylene oxides have, in particular, been proposed and have been put into practice as one group of ion conductive polymers. Batteries using ion conductive polymers in which electrolytic salts are dissolved in high polymer materials having such polyether structures have been widely described in patent documents (see, for example, U.S. Pat. Nos. 4,303,748, 4,589,197 and 4,547,440).
These ion conductive polymers, however, have low ion conductivities at a temperature below room temperature; therefore, reduction in size and weight and an increase in the energy density required for batteries for power sources to drive portable electronic devices and to back up memories cannot be implemented.
Therefore, a method for achieving an increase in the ion conductivities of these ion conductive polymers has been proposed. According to the proposed method, a monomer and an organic solvent (in particular, organic solvent having a high dielectric constant such as ethylene carbonate (EC) and propylene carbonate (PC)) are mixed so as to be polymerized and, thereby, a gel polymer electrolyte (hereinafter, referred to as “chemical crosslinked gel”) is obtained which maintain an electrolytic solution in a polymer network and which maintains a solid condition. The chemical crosslinked gel can greatly reduce the risk of liquid leakage. Therefore, it has become possible to utilize a laminate film wherein a metal foil and a resin film are stacked as an exterior material of batteries.
Manufacturing methods for such batteries are generally categorized into “stacked types” of manufacturing batteries (by stacking groups of electrodes) and “jelly-roll (winding) types” of manufacturing batteries (by winding positive electrodes, negative electrodes and separators in band forms).
So far, the mainstream of lithium ion batteries is the “jelly-roll types” of manufacturing batteries wherein positive electrodes, negative electrodes and separators are wound in band forms in order to emphasis importance on the productivity because the forms of batteries are restricted by the battery cans. On the other hand, it is easy to process the forms of lithium polymer batteries wherein laminate films are used as exterior materials. Therefore, manufacturing methods can freely be selected in order to achieve further reduction in weight and further freedom in the forms in addition to reductions in weight and increases in the capacity of lithium ion batteries and, for example, a stacked type can be adopted so that thin batteries having large areas can easily be manufactured.
In the case where polymer batteries are actually manufactured, the jelly-roll types are in the mainstream because facilities and manufacturing methods used for the manufacturing of conventional lithium ion batteries can be effectively utilized and the productivity is high. Accordingly, in the case where mechanization of production of polymer batteries are progressed for jelly-roll type batteries, only the batteries having similar forms to lithium ion batteries can ultimately be obtained due to the regulation of winding equipment for winding and peripheral jigs (the width of electrodes, winding core, and the like).
On the other hand, stacked type batteries in general have high freedom in form and have characteristics suitable for thin batteries, while mechanization of production is difficult due to the complication of their stacking processes. Although regulation of the sizes of positive electrodes, negative electrodes, and separator layers for separating between the positive electrodes and the negative electrodes has been proposed in order to increase the reliability (see, for example, Japanese Unexamined Patent Publication No. 2000-30742), the separator layers are set at sizes sufficiently larger than the positive electrodes and the negative electrodes in order to prevent short circuiting of the respective electrodes. Therefore, the problem of the complication of the stacking process is not solved. In addition, although there is an idea that separators are made to have a uniform size and both the positive and negative electrodes are covered with the separators so that the positioning can be easily carried out, a problem arises wherein the thickness of the battery increases and the energy density is reduced when the two electrodes are covered with the separators.
Furthermore, thin batteries having broad areas have problems that the batteries swell when gas is generated from the inside and that the batteries are weak against vibration in comparison with jelly-roll type batteries due to independence of electrodes.